Jill Mortimer swore the oath of allegiance to the Queen as she appeared in the chamber ahead of the Queen’s Speech debate.
Conservative MPs cheered her arrival in the Commons.
Ms Mortimer’s election to Parliament added another red wall seat to the list of Tory wins in traditional Labour areas.
Her victory was also the first by a Conservative candidate in the constituency since it was created almost 50 years ago.
The North Yorkshire farmer beat Labour candidate Dr Paul Williams, a GP and former MP for Stockton South.
Welcoming Ms Mortimer to the Commons and congratulating her for the win, Labour leader Sir Keir Starmer said: “She now has the huge honour of representing that great town. I hope she will forgive me if I say I hope for not too long.
“I wonder what plan she has for the 40ft inflatable of the Prime Minister?”
One Conservative MP could be heard joking “take it to Batley” – where the next by-election is expected.
Prime Minister Boris Johnson also welcomed Ms Mortimer and highlighted Conservative victories elsewhere before criticising Labour’s “bonkers” response.
Mr Johnson told MPs: “Labour’s response to these events is best summed up by the outgoing leader of Amber Valley Borough Council, who said these immortal words: ‘The voters have let us down, I hope they don’t live to regret it’.
“There you go, Labour’s bonkers solution in the face of any electoral setback is to wish they could dissolve the electorate and call another one, while we get on with our work taking forward our programme of change and regeneration filled with obligation to those we serve.”